© Confederation of Indian Industry
Variable Speed Drives
&
Harmonics
© Confederation of Indian Industry
Variable Speed Drives
� VSD
� Device used for varying the speed of
a driven equipment
� Applications
� Pumps, Fans & Blowers,
Compressors, conveyors etc.
© Confederation of Indian Industry
Why VSD’s ?
�Many Applications subject to
varying loads
� Due to Process Variations
� Changes in capacity Utilisation
� Conventional controls - High energy
loss
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� VSD – Advantages
� Accurate Speed Control – Exact
match to Process Requirement
� Energy Savings
� Productivity increase
� Low maintenance cost
� Smooth Starting
� Easy operation
Why VSD’s ?
© Confederation of Indian Industry
Types of VSD’S
� DC Drives
� Eddy Current Drives
� Rotor Resistance Control (GRR)
� Slip Power Recovery System (SPRS)
� Variable Fluid Coupling (VFC)
� AC Drives (VFD/VVVF)
© Confederation of Indian Industry
DC Drives
� Most Common VSD – Till AC VFD’s
� Advantage of DC Motors
� Good Speed Torque Characteristics
� Easy method of control - Armature voltage
control
� Wide speed range
� Low loss
� Smooth start
� Good dynamic response
� Applications – Steel, Paper, Cement, Sugar,
Tyre & Alloy Industries
© Confederation of Indian Industry
AC Drives
� AC motors are most common in industry
� AC Drives – Becoming popular
� Advantages over DC Motors
� No Carbon brushes, Commutator
� No regular Maintenance compared to DC
motors
� Can be fitted in Hazardous & adverse
conditions
© Confederation of Indian Industry
Eddy Current Drive
© Confederation of Indian Industry
Rotor Resistance Control
� Applicable only for Slip Ring
Induction Motors
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Rotor Resistance Control
© Confederation of Indian Industry
Rotor Resistance Control
� Higher rotor resistance
� Higher slip and lower speed
� Advantages
� Large speed control below rated
� No harmonics
� No electronic components
� Can be installed in any environment
© Confederation of Indian Industry
Rotor Resistance Control
� Disadvantages
� Power loss in the rotor resistance
� Cooling fans required – extra power
� Speed variation in steps
� Has lot of contactors
�Maintenance is required
� GRR loss = s(1 – s2) x Actual kW I/P
© Confederation of Indian Industry
SPRS ControlSlip- ring induction motor
Y
R
B
3-phaseinput
3-Phase Inverter Rectifier
Microcontroller
Firing Circuit
Actual speed
Speed feedback
Actual speed
Set speed
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SPRS Control
� Speed is controlled by drawing
power from rotor
� Advantages
� Slip power is recovered
� Disadvantage
� Limited speed control
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Speed Control in AC Motors
� Changing Supply Frequency
N = 120 f
P
� Variation in frequency – Speed Variation
� Keeping V/f Ratio constant
� Two different Poles – Winding
� 2 Pole / 4 Pole
� 4 Pole / 6 Pole
© Confederation of Indian Industry
AC Drives
� VFD
� Power electronic equipment
� Converts fixed AC supply into variable
AC supply
� Frequency & Voltage
© Confederation of Indian Industry
AC Drives
� AC Drives – Frequency Controller(V/f)
� Controlling Variable – Voltage ,
Frequency
� Torque Can not be controlled & only
speed can be varied
MModulator V/f ratioFreq reference
© Confederation of Indian Industry
AC Drives
� AC Drives – Flux Vector Control
� Indirect Method of torque control
� Closed loop Control
� Good torque response
� Accurate Speed Controls
� Full Torque from Zero Speed
MModulator Torque Control
Speed Control
Tacho
© Confederation of Indian Industry
Block Diagram of AC Drive
Rectifier
Bridge
FILTER
InverterWith
VoltageControl
LOAD
AC DRIVESYSTEM
3 φφφφSUPPLY
© Confederation of Indian Industry
.
What is a frequency converter ?VFD
Control
V & FSpeed
Reference
RectifierDC Link
Choke
415V,50Hz
Input
IGBT
Output
Stage
Motor
Output0-415V,0-50Hz
Fixed AC - DC- Filter DC -Variable AC
© Confederation of Indian Industry
AC Drives
� Advantages
� Soft Starting
� Precise speed & Torque control
� Wide Speed range
� High reliability
� Low noise
� Capability for speed reversal /
regenerative braking
� Energy saving
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Technical Specifications - VSDs
System Layout
Conditions - Equipment Operate
System Requirements
© Confederation of Indian Industry
� Input supply range
� Motor rating
� Application
� Type of load
� Speed range
� Environmental conditions
� I/O requirements
� Motor cable length
VFD Selection Criteria
© Confederation of Indian Industry
� VFDs are available in the voltage range – 220V, 380-500V and 525-690V
� All VFDs can take care of supply frequency range
� Note down motor nameplate details� Voltage, kW & Current
� Frequency
� RPM
� Current
VFD Selection Criteria – Input Supply
© Confederation of Indian Industry
� Select VFD that can give output suitable to motor - This is the minimum size of VFD required
� Application to be studied� Constant torque or variable torque
� VFDs with 110% overload are suitable for variable torque � 150% overload for constant torque applications
VFD selection criteria – Motor rating
© Confederation of Indian Industry
� Speed-torque characteristics to know the initial torque requirement
� Select VFD accordingly
� Does the application demand open loop OR closed loop operation ?
� Most VFDs will work in open loop
� Performance with closed loop device connected to motor ?
VFD selection criteria –Application
© Confederation of Indian Industry
� If the application needs process
feedback to be taken in VFD
� It should have minimum 2 analog
inputs…..one for reference and
second for feedback
� VFD should have PID loop
� E.g. pressure control
VFD selection criteria –Application
© Confederation of Indian Industry
� Depending on where the VFD will be mounted, select VFD with proper IP class of protection� VFDs are available with IP00, IP20, IP21, IP54 and IP66 protection class
� VFDs are available with 40oC, 45oC and 50oC operating ambient temperature� Select VFD appropriate to the ambient
VFD selection criteria –Environment
© Confederation of Indian Industry
� VFD with 40oC ambient temp needs to be derated while operating at 45 or 50oC
� VFDs pollute the electrical environment by injecting harmonics in the electrical system
� Select VFD which has inbuilt DC or AC choke� This reduces % of harmonics injected
VFD selection criteria –Environment
© Confederation of Indian Industry
� VFD manual specifies maximum cable length between VFD and motor� Select VFD which can support the installation to avoid additional hardware in the circuit
� Automatic Energy Optimization feature will reduce the consumption of the power at light loads
VFD selection criteria – Cable length
© Confederation of Indian Industry
Maximum Cable Length w/o O/P Chokes
300 m120 kw
200 m45-110 kw
150 m15-37 kw
100 m5.5-11 kw
60 m4.0 kw
50 m2.2 kw
40 m1.5 kw
30 m1.1 kw
25 m0.75kw
© Confederation of Indian Industry
� Select VFD which has innovative cooling concept to reduce the air conditioner load
VFD selection criteria – Others
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Precautionary Measures &
Trouble Free Operation ofVSDs
© Confederation of Indian Industry
Physical Installation Procedure -Electronic VSDs
Drives - Not to be installed
� Direct Sunlight� High Temperature� High Humidity� Excessive Vibration
© Confederation of Indian Industry
Ambient Conditions - Drives
Dust Free
Corrosion Free
Temperature < 40° C
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Operation of VSDs :
Reliable
Trouble-free
Substantial Energy Savings
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Impact of Harmonics
© Confederation of Indian Industry
= ++
What are Harmonics ?
Resultant Wave Fundamental 3rd Harmonic 5th Harmonic
© Confederation of Indian Industry
HARMONICS
� Normal Supply system operates at 50 Hz
� The Harmonics are sinusoidal waves that
are integral multiples of fundamental
frequency (150 Hz, 250Hz, 350Hz ...)
� The overall impact of Voltage/ current
harmonic distortion on a power system
wave form is called as Total Harmonic
Distortion(THD)
� Pollution in Electrical System
© Confederation of Indian Industry
Non Linear Non Linear Non Linear Non Linear
LoadLoadLoadLoad
HarmonicI
Voltage to other
circuits and the
pollution
circulates
Harmonics
Generation
Non Linear Loads
draw non linear
Current From a
linear source
When Linear Voltage is
applied to
Distorts voltage
© Confederation of Indian Industry
Sources of Harmonics
� Non-linear Loads
� Static Switches - Diodes, SCR’s,
GTO’s, Transisters, IGBT’s etc.
� Devices / Equipment
� Variable Speed AC & DC Drives
� Arc Furnaces
� Induction Furnaces
� Welding sets
� UPS
© Confederation of Indian Industry
Effect of Harmonics
� Power Distribution System
� Additional Losses in wires & cables
� Extra heating/noise of Transformers
� Circuit breaker & Protective Relays malfunction
� Other Equipment
� Erratic operation of computers, Telecommunication, Video Monitors & Electronic test equipment
� Failure of Capacitors
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Effect of Harmonics
� Other Equipment
� Overheating of Motors
� De-rating of Generators
� Malfunction of Measuring instruments
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Problems created by harmonics
� Excessive harmonic currentmay lead to overheating (or even burning) of network components
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Problems created by harmonics
�Capacitor
problems
Due to its lower
impedance, capacitors
are even more
susceptible to higher
order harmonics
© Confederation of Indian Industry
Harmonics Distortion
� Voltage Harmonic
� Distortion in Voltage Waveform
� More harmful than Current
Harmonics
� Electrical equipment sensitive to
Voltage harmonics
� Current Harmonics
� Distortion in Current waveform
� Load Dependent
© Confederation of Indian Industry
Total Harmonic Distortion
%100)(
)(1
2
xV
VVTHD
n∑=
Where
n = 2 …. 31Vn = Harmonic Voltage of nth orderV1 = Fundamental voltage
© Confederation of Indian Industry
Acceptable Limit -International Guidelines
Supply SystemVoltage (kV) atpoint of common
coupling
TotalHarmonicVoltage
Distortion VT(%)
IndividualHarmonicVoltage
Distortion (%)
Odd Even0.415 5 4 2
6.6 and 11 4 3 1.75
33 and 66 3 2 1
132 1.5 1 0.5
Harmonic Voltage Distortion limits at any point on the system
IEEE G.5/3 Sept. 1976 : Limits for Harmonics.
© Confederation of Indian Industry
Harmonic Distortion - IEEE Norm
� Voltage Distortion
� 415 Volts : < 5% THD
� 33 KV : < 3% THD
� Current Distortion
� Depends on Isc/IL ratio of bus
� Generally less than 15%
�Voltage Harmonics - More harmful to electrical equipment
�Motors, Capacitors etc.
� Current Harmonics – Magnetic interference and increase in distribution losses
© Confederation of Indian Industry
Options for Reducing Harmonic Distortion
� Use of PWM AC Drives
� Use of drive with effective DC link
filtering
� Use of 12 - pulse rectifier drive
� Installation of Harmonic filters
© Confederation of Indian Industry
Harmonic Filters
� Two types of filters
� Active filters
� Passive filters
� Active filters
� Introduces signals in the opposite
direction to compensate harmonic
distortion
� Costly
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Harmonic Filters
� Passive filters
� Built with passive components
� Capacitors & Reactors
� Economical
� Do not eliminate harmonics
completely
© Confederation of Indian Industry
Harmonic Filters
� Benefits of filters
� Reduced system losses
� Improved system protection
� Reduced neutral to ground voltage
� Improved PF of non linear loads
© Confederation of Indian Industry
Harmonic Study
� Observe any malfunction
� If yes, List & Quantify non-linear
Loads
� Conduct Preliminary Harmonic
Study
� Compare with Standards
� Detailed study & Cost economics
� Install Harmonic filters
© Confederation of Indian Industry
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